Effects of nonnative stream fishes and riparian disturbances that alter flows of resources between stream and riparian food webs in northern Japan (2002-2004)
Habitat degradation and invasions by nonnative species, the two leading causes of biodiversity loss, are prevalent in streams worldwide. We conducted a large-scale field experiment and two field observational studies in Hokkaido, Japan to measure the effects of riparian deforestation, channel straightening (channelization), and nonnative rainbow trout (Oncorhynchus mykiss) invasion on linked stream-riparian forest food webs. Recent research has shown that invertebrate prey "reciprocal subsidies" from each ecosystem to the other supply about 25-50% of the energy requirements of consumers in the recipient ecosystem (e.g., fish in streams, birds in the riparian forest). Therefore, we predicted that each disturbance we studied could reduce the flux of prey across the terrestrial-aquatic ecosystem boundary and alter the growth or abundance of subsidized consumers. In the field experiment, cutting off the input of terrestrial prey to the stream with a mesh greenhouse (mimicking one aspect of riparian deforestation) caused native Dolly Varden charr (Salvelinus malma) to switch to feeding on benthic grazing insects, which resulted in increased algae but also reduced emergence of adult aquatic insects. The greenhouse prevented this export of insect emergence and reduced riparian spider density by 83%. Surprisingly, adding nonnative rainbow trout, which usurped most terrestrial prey, had the same effect on charr, the stream food web, and insect emergence, which caused a 65% decline in riparian spiders. A companion field study in six other streams to test the generality of these results showed that rainbow trout also usurped most terrestrial prey and reduced native charr abundance by more than 75%. A second field study showed that stream channelization or riparian vegetation loss alone, or both combined, resulted in similar low riparian spider density compared to control sites, even though vegetation that spiders use for web sites had grown back at previously channelized sites. Channelization is known to reduce benthic insect abundance, and likely reduced emergence that supported riparian spiders. Together, these results indicate the critical importance of reciprocal subsidies of invertebrate prey between streams and riparian forests in sustaining both terrestrial and aquatic biodiversity in these linked ecosystems.
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Baxter, C.V., K.D. Fausch, M. Murakami and P.L. Chapman. 2007. Invading rainbow trout usurp a terrestrial prey subsidy to native charr and alter their behavior, growth, and abundance. Oecologia 153:461-470.
Baxter, C. V., K. D. Fausch, and W. C. Saunders. 2005. Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones. Freshwater Biology 50(2):201-220.
Laeser, S.R., C.V. Baxter and K.D. Fausch. 2005. Effects of stream channelization and riparian vegetation loss on web-weaving spiders in northern Japan. Ecological Research. 20:646-651.
Baxter, C.V., K.D. Fausch, M. Murakami and P.L. Chapman. 2004. Non-native stream fish invasion restructures stream and riparian forest food webs by interrupting reciprocal prey subsidies. Ecology 85: 2656-2663.